Recently, as structure, properties and so forth of devices such as electronic or optical devices have become more advanced, a precise uniformity of the film thickness and composition, etc., of a semiconductor epitaxial growth film, which is a base of such a device, have been required.
Compound semiconductor crystals have hitherto been formed by a liquid epitaxial growth method, but this growth method is hard of control due to a high growth rate, and thus it has been difficult to satisfy recent, advanced requirements.
For this reason, attention is directed to an MOVPE process in which a semiconductor crystal is grown by pyrolyzing an organometallic compound and a hydride in a vapor phase, as a technique for epitaxially growing a uniform compound semiconductor crystal on a large area substrate or a plurality of substrates, for the liquid epitaxial growth process.
The furnaces of the MOVPE crystal growth are roughly classified into a horizontal CVD furnace and a vertical CVD furnace.
FIGS. 1 to 3 are illustrations showing the principal horizontal furnaces; in the drawings, 1 is a gas injector serving as an introducing port for introducing a deposition gas, 2 is a reaction tube, 3 is a substrate for deposition, 4 is a susceptor supporting the substrate 3, and 5 is a gas exhaust port.
FIG. 1 shows a typical horizontal CVD furnace.
In the horizontal furnace of this drawing, a substrate 3 is horizontally placed on a susceptor 4 installed on a reaction tube 2, and a deposition gas containing raw materials for deposition is substantially horizontally supplied to the substrate 3.
FIG. 2 shows a barrel-type furnace, as one of the horizontal CVD furnaces.
In the horizontal furnace of this drawing, a plurality of substrates 3 are placed on the side faces of a susceptor 4 provided in a reaction tube 2, and a deposition gas is supplied from above the reaction tube 2 substantially in parallel to the substrates 3. The susceptor 4 is then rotated so that variations among the crystals deposited on the substrates 3 do not occur. The individual substrates 3 are also rotated, to produce a uniform deposition on a surface of the substrate.
FIG. 3 shows a planetary-type furnace, as another of the horizontal CVD furnaces.
In the horizontal furnace of this drawing, a plurality of substrates 3 are placed on a susceptor 4 horizontally provided in a reaction tube 2, and a deposition gas introduced from above the reaction tube 2 is allowed to horizontally flow along the susceptor 4 from substantially the center of the susceptor 4, and is supplied to the substrates 3.
These horizontal furnaces are commonly used when epitaxially growing GaAs for high speed devices and AlGaAs/GaAs systems for short wavelength optical devices by an MOVPE process, as the gas flow is relatively simple and the deposition on a large area substrate or a plurality of substrates is facilitated due to the construction thereof.
Nevertheless, the horizontal furnaces have the following three major problems:
First, since a deposition gas is unidirectionally introduced along a substrate surface, the horizontal furnaces have a problem with the growth of crystals on the substrate surface, i.e., the consumption of the deposition gas, the concentration of the deposition gas becomes thin in the direction from upstream to downstream, and as a result, the crystal deposition rate on the surface of the deposition substrate is slower in the direction from upstream to downstream of the gas flow.
Second, a problem arises in that a deposition gas introduced into a horizontal furnace has a distribution in the temperature of the deposition gas on a substrate, because the gas is gradually heated on the substrate in the direction of from upstream to downstream of the gas flow. This has resulted in a distribution of the decomposition condition of the deposition gas on the substrate, as well as a distribution of the composition of the deposited crystal on the substrate.
Third, a problem arises in that, in the horizontal furnaces, a ceiling or wall of a reaction tube exists in the vicinity of a deposition substrate, and such a ceiling or wall has an effect on the deposition of the crystal on the substrate. Namely, reaction by-products are deposited on the ceiling or wall of the reaction tube during the repetition of the crystal deposition therein, which vary the degree of decomposition of the deposition gas and have an effect on the film thickness and composition of the crystal deposited on the substrate.
FIGS. 4 and 5 illustrate the above first problem, wherein FIG. 4 shows examples of isoconcentration lines of a gas on a substrate. These isoconcentration lines indicate that the deposition gas concentration becomes thinner and thinner as it flows further from upstream to downstream over the substrate.
FIG. 5 shows the effect due to the first problem of the distribution of the deposition gas concentration on a substrate, and exemplifies a distribution of a film thickness in a horizontal furnace. The abscissa axis is the distance from the upstream of the substrate, and the ordinate axis is the thickness of the deposited film. As can be seen in this drawing, the thickness of the film deposited on the substrate becomes thinner and thinner from upstream to the downstream of the substrate.
This first problem, however, can be solved by making the crystal deposition rate uniform to a certain degree by rotating the deposition substrate. In addition, the second and third problems have not appeared in the case of binary compound semiconductors and ternary compound semiconductors consisting of two group III elements and one group V element, and therefore, have not had very significant effects on the deposition of AlGaAs/GaAs system crystals.
Nevertheless, in the deposition of a quaternary compound semiconductor such as InGaAsP system crystals used for optical devices of the 1 micrometer wavelength band or AlGaInP systems used in visible light lasers, the second and third problems have a particularly significant influence.
First, the temperature distribution of a gas flow on a substrate has a significant effect on the solid phase composition of group V elements in the InGaAsP, since the thermal decomposition efficiencies of AsH.sub.3 (arsine) and PH.sub.3 (phosphine) of the group V raw materials commonly used in an MOVPE deposition of InGaAsP differ greatly depending on a temperature. Therefore, the second problem in the horizontal furnaces is fatal to the deposition of the crystal of a quaternary compound semiconductor such as InGaAsP.
FIG. 4 also represents isothermal lines on a substrate. In this case, the lines show that the deposition gas has a higher and higher temperature as it flows further from upstream to downstream over the substrate.
FIG. 6 is a graph showing an effect due to a temperature distribution of a gas flow on a deposition substrate in the deposition of an InGaAsP system crystal, and illustrates a composition distribution in a horizontal furnace. In this drawing, the abscissa axis is the distance from the upstream of the deposition substrate, and the ordinate axis is the photoluminescence (PL) wavelength of the deposited InGaAsP crystal. A PL wavelength is a wavelength of light proper to a substance generated corresponding to the band gap of the substance when irradiated by a certain light.
In the case of an InGaAsP crystal, this graph may be considered to substantially correspond to the composition ratio of As/P of group V elements, because it has been verified that In and Ga of group III elements are deposited in an approximately uniform composition. This graph shows that the crystal deposited on the substrate has a composition in which As is decreased and P is increased from upstream to downstream. (A crystal containing more As has a large PL wavelength than a crystal containing more P.)
Also, for the above third problem, it has been confirmed, by experiments, etc., that deposits on the ceiling or wall of a reaction tube have significant effects on the uniformity of the group III solid phase composition, particularly the composition of In and Ga.
From the above, it can be considered that the use of a horizontal furnace causes difficulty in obtaining a uniform deposition of a III-V compound semiconductor crystal containing In and Ga or As and P together (such as InGaAsP, AlGaInP).
On the other hand, the problems as mentioned above do not exist in vertical furnaces, in principle.
FIG. 7 shows a typical vertical CVD furnace, wherein the same items as in FIG. 1 are designated by the same numerical signs.
In this vertical furnace, a substrate 3 is mounted on a susceptor 4 horizontally provided in a reaction tube 2, and a deposition gas introduced from a gas injector 1 provided above the reaction tube 2 is vertically supplied to the surface of the substrate 3.
A deposition gas is horizontally supplied to a substrate in the case of a horizontal furnace, but in a vertical furnace, since a deposition gas is vertically supplied to a substrate, the above mentioned first and second problems do not occur, in principle, if an ideal gas flow is realized. The third problem also does not occur because of the construction of the furnace, as no ceiling or wall of a reaction tube exists in the vicinity of the surface of a substrate.
In vertical furnaces, it is ideal to supply a gas having an equal concentration to the entire surface of a deposition substrate at an equal rate. Since the entire surface of the substrate can be in the same condition if this condition is satisfied, a crystal having a uniform film thickness and a uniform composition can be deposited on the substrate.
In general, however, it is extremely difficult to bring the diameter of a gas injector (commonly less then 1 centimeter) close to the diameter of a substrate (generally 5 to 8 centimeters), and if this is achieved, it is difficult to supply a deposition gas having a uniform concentration from a gas injector with a large diameter. In practice, a gas injector having a smaller diameter than the diameter of a substrate therefore must be provided above the center of the substrate. Consequently, the deposition gas introduced from the gas injector is concentrated at the central portion of the substrate, as shown in FIG. 7.
FIG. 8 illustrates the isoconcentration and isothermal lines of a gas on a substrate in a prior vertical furnace. In this drawing, the nearer the lines approach the substrate, the lower the concentration and the higher the temperature.
It has been found that the isoconcentration and isothermal lines vary greatly at the center of the substrate, as shown in the drawing, since the deposition gas is concentrated at the central portion of the substrate, as aforementioned.
FIG. 9 is a graph illustrating a film thickness distribution of an InGaAsP crystal deposited on a substrate in a prior vertical furnace. As a consequence of the large variation of gas concentration near the center above a substrate as shown in FIG. 8, the film thickness of a crystal deposited on the substrate has a distribution which becomes maximum near the center.
Similarly, FIG. 10 is a graph illustrating a composition distribution of an InGaAsP crystal deposited on a substrate in a conventional vertical furnace. Again, in this drawing, the As/P composition ratio can be determined by detecting a PL wavelength in the face of a substrate, as in FIG. 6. It is found that the crystal deposited on the substrate has a composition distribution with large variations, as a consequence of the large variation of gas temperatures near the center above the substrate.
Furthermore, in the vertical furnaces, a convection of a deposition gas occurs in a reaction tube as shown in FIG. 7, since the concentrated deposition gas is supplied to the central portion of a substrate as aforementioned. Therefore, the uniformity of the film thickness or composition of a crystal deposited on the substrate is subjected to variations due to the convection as well.
When a heterojunction, for example, is formed on a substrate, the abruptness of the heterointerface is adversely affected.
To improve the concentration of a deposition gas at the center of a substrate, as described above, the flow rate control technique may be used as proposed by the inventors in Japanese Unexamined Patent Publication No. 1-140712. In this technique, a plurality of sub-injectors are provided in such a manner that they face a substrate and are arranged along a centerline in the substrate plane, and a gas controlled at a given flow rate is supplied from each sub-injector toward the surface of the substrate being rotated.
Using this technique, both a film thickness and a composition can have an improved uniformity, as far as the deposition of crystal of a binary compound semiconductor such as GaAs or a ternary compound semiconductor such as GaInAs is concerned, but when this technique is applied to the deposition of crystal of a quaternary compound semiconductor such as InGaAsP, the uniformity of the film thickness is good, but no improvement is found in the uniformity of the composition (particularly, the uniformity of the composition for As and P of group V elements in the deposited crystal), because in this case, only a part of the substrate is vertically supplied with a deposition gas directly from sub-injectors arranged along a centerline in the substrate.
That is, with the technique described in Japanese Unexamined Patent Publication No. 1-140712, since the deposition gas supplied from sub-injectors strikes vertically against the portion of a substrate directly below the sub-injectors, and then the flow direction is laterally turned and flows along the surface of the substrate and toward the edge thereof, the gas is heated in the direction from upstream to downstream of the lateral flow, leading to the distribution of the gas temperature on the substrate. Accordingly, although this technique was useful for supplying a deposition gas in an even concentration onto a substrate, it is still insufficient for making the temperature distribution of the gas flow as uniform as possible to hold a constant ratio of deposition rate among group V elements.